TITLE:   GCN CIRCULAR
NUMBER:  45969
SUBJECT: EP261015a: refined analysis of EP-FXT observations
DATE:    26/10/17 05:09:17 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

R.-Z. Li (YNAO), Z.-X. Li, G.-L. Huang (IHEP), M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:

The X-ray transient EP261015a was detected by the Wide-field X-ray Telescope (WXT) onboard the Einstein Probe (EP) mission at 2026-10-15T13:15:36 UTC (Li et al., GCN 45956). Follow-up observations with the Follow-up X-ray Telescope (FXT) onboard EP began at 2026-10-16T04:31:12 UTC, approximately 15.26 hours after the WXT trigger, with an exposure time of 3231 s.

We stacked the FXT-A and FXT-B data. Within the EP/WXT error circle (Li et al., GCN 45956), no X-ray source was detected at the position of the optical candidate (Corcoran et al., GCN 45955; Méndez et al., GCN 45960; Ma et al., GCN 45963; Mohan et al., GCN 45967). The derived 0.5–10 keV flux upper limit is approximately 1.33 × 10^-13 erg cm^-2 s^-1 (90% confidence level).

An uncatalogued X-ray source was detected in the stacked data within the EP/WXT error circle (Li et al., GCN 45956). Its position is R.A. = 44.3459 deg, Decl. = −13.3915 deg (J2000), with a positional uncertainty of 10 arcsec in radius (90% confidence level, including statistical and systematic uncertainties). The 0.5-10 keV FXT spectrum of this source can be fitted with an absorbed power-law model, with the Galactic hydrogen column density fixed at 3.5 × 10^20 cm^-2 and a photon index of 1.55 (+1.45, −1.43). The derived unabsorbed 0.5–10 keV flux is 7.01 (+6.27, −3.45) × 10^-14 erg cm^-2 s^-1. All quoted uncertainties on the spectral parameters and fluxes are at the 90% confidence level.

Launched on January 9, 2024, EP is a space-based X-ray observatory designed to monitor the soft X-ray sky and perform X-ray follow-up observations (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).